Well, if Holy had the sense to spare humanity, and that is the planet's ultimate will, one would think Genesis would follow it. I mean, he is Minerva's protector/bodyguard/bitch-boy/whatever, so maybe Minerva wouldn't be as loose with him as she was with the Weapons?
As for Sephiroth, I honestly want him to stay dead.
Not because I dislike him at all, but I think that his and Cloud's story...is kinda done? I think they had a good closure in AC/C. Cloud finally standing up to the biggest big bad on will alone and coming up victorious, earning the right to live with his family and exterminating his past sins.
I know they left a gigantic bait for return in the end, but...
If you're gonna do anything else, the only other thing would be something ridiculous like...Cloud kills himself to neutralize Sephiroth's will in the Lifestream forever.
What?
He's the only other being in the planet who can take a swim in the Lifestream it'self, the great current which runs under the crust, and stay completely fine. His will has proven to be stronger than Sephiroth's, so if he's there, Sephiroth can't do shit - unlike Aerith's will, who turns around and runs off at the sight of Seph.
Of course, therein lies the problem. Cloud's development and journey is nothing without his happy ending life with Tifa and the kids, so yeah. Stupid.